Large Ion Currents Research Articles

Plasma properties conditioned by the magnetic throat location in a helicon plasma device

Measurements are taken using a helicon plasma device to analyze the spatial distribution of plasma properties as the throat of the magnetic nozzle is axially shifted with respect to the antenna center. Krypton plasma is generated in the sub-kilowatt range and probed using a suite of diagnostics including a rf-compensated Langmuir probe, a planar probe, and laser-induced fluorescence. It is found that larger ion currents and increased plasma confinement are achieved when the throat of the magnetic nozzle is located downstream the antenna center, at a distance that equals or exceeds two times the antenna length. The ions, although being accelerated, retain subsonic velocities even beyond the magnetic throat.

Gain-of-Function of TMEM16E/ANO5 Scrambling Activity Caused by a Mutation Associated with the Bone Genetic Disease Gnathodiaphyseal Dysplasia

Members of the TMEM16 or anoctamin membrane protein family are involved in a variety of functions that include ion transport, phospholipid scrambling and regulation of other membrane proteins. In particular, TMEM16A, 16B and 16F have a clear plasma membrane localization but while 16A and 16B work as ‘pure’ ion channels, TMEM16F shows both ion channel and scramblase activities. Recent data suggest that also TMEM16E, the closest relative of TMEM16F, may work as a phospholipid scramblase, although a direct confirmation is missing. TMEM16E is highly expressed in bone tissue and skeletal muscle and different mutations in the human TMEM16E (ANO5) gene are associated with the bone disease gnathodiaphyseal dysplasia (GDD) or to different forms of muscular dystrophy (LGMD2L, limb-girdle muscular dystrophy-2l and distal MMD3, miyoshi muscular dystrophy-3). Our data show that human TMEM16E, when overexpressed in mammalian cell lines, displays partial plasma membrane localization and give rise to phospholipid scrambling as well as non-selective ionic currents with slow time-dependent activation at highly depolarized membrane potentials and in presence of high intracellular Ca2+. We address the effect of the GDD-causing mutation T513I, located in the second extracellular loop, on TMEM16E and we show phospholipid scrambling activity and large time-dependent ion currents even at low cytosolic Ca2+ concentrations. Our data provide the first direct demonstration of Ca2+-dependent current and phospholipid scrambling activity for TMEM16E and suggest a gain-of-function phenotype related to a GDD mutation.

Gain of function of TMEM16E/ANO5 scrambling activity caused by a mutation associated with gnathodiaphyseal dysplasia

Mutations in the human TMEM16E (ANO5) gene are associated both with the bone disease gnathodiaphyseal dysplasia (GDD; OMIM: 166260) and muscle dystrophies (OMIM: 611307, 613319). However, the physiological function of TMEM16E has remained unclear. We show here that human TMEM16E, when overexpressed in mammalian cell lines, displayed partial plasma membrane localization and gave rise to phospholipid scrambling (PLS) as well as non-selective ionic currents with slow time-dependent activation at highly depolarized membrane potentials. While the activity of wild-type TMEM16E depended on elevated cytosolic Ca2+ levels, a mutant form carrying the GDD-causing T513I substitution showed PLS and large time-dependent ion currents even at low cytosolic Ca2+ concentrations. Contrarily, mutation of the homologous position in the Ca2+-activated Cl− channel TMEM16B paralog hardly affected its function. In summary, these data provide the first direct demonstration of Ca2+-dependent PLS activity for TMEM16E and suggest a gain-of-function phenotype related to a GDD mutation.

Anoctamin 6 mediates effects essential for innate immunity downstream of P2X7 receptors in macrophages.

Purinergic P2X7 receptors (P2X7R) are fundamental to innate immune response. In macrophages, transient stimulation of P2X7R activates several transport mechanisms and induces the scrambling of phospholipids with subsequent membrane blebbing and apoptosis. These processes support phagocytosis and subsequent killing of phagocytosed bacteria. Here we demonstrate that the stimulation of P2X7 receptors activates anoctamin 6 (ANO6, TMEM16F), a protein that functions as Ca(2+) dependent phospholipid scramblase and Ca(2+)-activated Cl(-) channel. Inhibition or knockdown of ANO6 attenuates ATP-induced cell shrinkage, cell migration and phospholipid scrambling. In mouse macrophages, Ano6 produces large ion currents by stimulation of P2X7 receptors and contributes to ATP-induced membrane blebbing and apoptosis, which is largely reduced in macrophages from Ano6-/- mice. ANO6 supports bacterial phagocytosis and killing by mouse and human THP-1 macrophages. Our data demonstrate that anoctamin 6 is an essential component of the immune defense by macrophages.

Plasmodium berghei-infection induces volume-regulated anion channel-like activity in human hepatoma cells

Parasite infection can lead to alterations in the permeability of host plasma membranes. Presented here is the first demonstration that this phenomenon occurs in Plasmodium-infected liver cells. Using the whole-cell patch-clamp technique, volume-regulated anion channel (VRAC) activity was characterized in Huh-7 cells (a human hepatoma cell line) before and after infection with Plasmodium berghei. Consistent with the presence of VRACs, hypotonic bath solution induced large ion currents in Huh-7 cells that rectified outwardly, reversed close to the equilibrium potential for Cl- and were inhibited by tamoxifen, clomiphene, mefloquine and 5-nitro-2, 3-(phenylpropylamino)-benzoic acid (NPPB), with IC50 values of 4 ± 1, 4 ± 2, 2 ± 1 and 52 ± 12 μM respectively. In isotonic conditions, initial current recordings measured in uninfected and immature (24 h post invasion) parasite-infected Huh-7 cells were similar (with conductances of 14 ± 3 versus 19 ± 5 pS/pF). However, in mature (48–72 h post invasion) parasite-infected Huh-7 cells there was a sevenfold increase in currents (with a conductance of 98 ± 16 pS/pF). The elevated currents observed in the latter are consistent with VRAC-like activity and the possible reasons for their activation are discussed.

Human FXYD2 G41R mutation responsible for renal hypomagnesemia behaves as an inward-rectifying cation channel

A mutation in the human FXYD2 polypeptide (Na-K-ATPase gamma subunit) that changes a conserved transmembrane glycine to arginine is linked to dominant renal hypomagnesemia. Xenopus laevis oocytes injected with wild-type FXYD2 or the mutant G41R cRNAs expressed large nonselective ion currents. However, in contrast to the wild-type FXYD2 currents, inward rectifying cation currents were induced by hyperpolarization pulses in oocytes expressing the G41R mutant. Injection of EDTA into the oocyte removed inward rectification in the oocytes expressing the mutant, but did not alter the nonlinear current-voltage relationship of the wild-type FXYD2 pseudo-steady-state currents. Extracellular divalent ions, Ca2+ and Ba2+, and trivalent cations, La3+, blocked both the wild-type and mutant FXYD2 currents. Site-directed mutagenesis of G41 demonstrated that a positive charge at this site is required for the inward rectification. When the wild-type FXYD2 was expressed in Madin-Darby canine kidney cells, the cells in the presence of a large apical-to-basolateral Mg2+ gradient and at negative potentials had an increase in transepithelial current compared with cells expressing the G41R mutant or control transfected cells. Moreover, this current was inhibited by extracellular Ba2+ at the basolateral surface. These results suggest that FXYD2 can mediate basolateral extrusion of magnesium from cultured renal epithelial cells and provide new insights into the understanding of the possible physiological roles of FXYD2 wild-type and mutant proteins.

Preparative Separation of Mixtures by Mass Spectrometry

A specially designed mass spectrometer which allows for preparative separation of mixtures is described. This mass spectrometer allows for large ion currents, on the order of nanoamperes, to be produced by electrospray and transmitted into a high vacuum. Accumulation of nanomole quantities of collected and recovered material in several hours is demonstrated. The use of high-velocity ions reduces space charge effects at high ion currents. Separation of mass occurs simultaneously for all ions, providing a 100% duty cycle. The use of a linear dispersion magnet avoids compression at higher m/z ratios. A deceleration lens slows the ions to allow for soft landing at low kinetic energy. The ions are neutralized by ion pairing on an oxidized metal surface. Retractable landing plates allow for easy removal of the separated components.

Selective removal of matrix ion peaks in plasma source time-of-flight mass spectrometry: ion deflection and detector gating

A significant problem in plasma source time-of-flight mass spectrometry (TOFMS) is the saturation of detectors due to plasma gas and matrix ions. This paper outlines two experimental procedures to minimize the detector response to large ion currents in a glow discharge time-of-flight mass spectrometer. One novel solution is a deflection device that includes two parallel plates for ion ejection and two sleeve plates (after the deflection plates) to reduce ion and neutral scattering. Another approach is to gate the detector, by lowering the voltage on the microchannel plates, when an intense ion pack is expected. Both approaches are efficient in eliminating the plasma gas peaks and matrix ions in plasma source time-of-flight mass spectrometry. The matrix ion intensity can be reduced by several orders of magnitude, with minimal influence on the analyte ions. The detector gating technique requires a high peak current pulse with a flexible rise time. However, this approach will cause an electrical ringing to be superimposed on the analytical signal. Therefore, ion deflection is the method of choice for plasma TOFMS.

Development of a novel structure zone model relating to the closed-field unbalanced magnetron sputtering system

It is well established that the microstructure of a thin film strongly influences its physical and chemical properties. Microstructure, in turn, is determined by a number of deposition and process parameters which control the energy delivered to the growing film. The closed-field unbalanced magnetron sputtering (CFUBMS) process has now been developed to the stage where it can be routinely used to deposit very high quality, well adhered coatings of a wide range of metals and ceramics. A key factor in the success of this process is the ability to transport large ion currents to the substrate. This can enhance the formation of fully dense coating structures at relatively low homologous temperatures, compared to other sputtering systems. The importance of microstructure on the performance of a coating has led to the development of models designed to describe coating structure in terms of specific deposition parameters. Several such structure zone models (SZMs) relating to various physical vapor deposition (PVD) processes have been published. However, because of the advantages of operating in the CFUBMS mode, the structure of coatings deposited in this mode do not conform to those predicted by existing SZMs relating to other PVD processes. Also, in most existing SZMs, the final coating structure is described in terms of the homologous temperature of the coating and one other parameter which attempts to describe the additional influence on the structure of the simultaneous ion bombardment of the growing film. Several parameters have been used to fill this role including coating pressure, substrate bias voltage, and an energy parameter defined as the average energy carried by the arriving ions per condensing atom. However, other studies have shown that ion energy and ion flux are fundamental parameters in ion-assisted PVD processes and their effects must be considered separately when describing coating structures. A detailed investigation has now been carried out into the CFUBMS process. As a result of this, a SZM relating to the CFUBMS system has been developed, in which coating structures are described in terms of homologous temperature, bias voltage and the ion-to-atom ratio incident at the substrate. This is a novel model which allows the influence of ion flux and ion energy to be considered separately. This study has also highlighted a number of other characteristics of the CFUBMS system. For example, both ion current density and deposition rate are directly proportional to the target current, although their coefficients of proportionality differ. Deposition rate decreases more rapidly with increasing substrate-to-target separation than ion current. Consequently the ion-to-atom ratio incident at the substrate increases with separation. Indeed, with magnetrons of fixed magnetic configuration, in order to increase the ion-to-atom ratio for any set of deposition parameters, it is necessary to increase the substrate-to-target separation.

Necessity and feasibility of spacecraft potential control in the Hermean magnetosphere

During its passages through the magnetosphere of Mercury the electron detector on board the Mariner 10 spacecraft provided first evidence of significant positive charging of the spacecraft of the order of 100V positive. At the same time the disadvantages of lacking spacecraft potential control became evident through the loss of information on the low energy part of the plasma distribution functions. This experience and model calculations lead to the conclusion that provisions to keep the potential at a low level are highly desirable, if not mandatory, for satisfactory measurements in the plasma environment of Mercury. The required charge for spacecraft potential control can be provided by emitters of positive ions, as demonstrated by an instrument on the Geotail spacecraft. This paper formulates the scientific and technical requirements on a spacecraft potential control system in the space environment of Mercury. The results of a feasibility study for liquid metal ion emitters in a design which represents a further development of the instruments used on Geotail and in preparation for the Cluster and Interball spacecraft are also presented. It is demonstrated that the large ion currents required for a mission to Mercury can be supplied by moderate mass and power resources.

Electron cyclotron resonance plasma reactor for cryogenic etching

Electron cyclotron resonance (ECR) plasma reactors are being used for ultralarge scale integrated circuit fabrication to meet the stringent requirements on submicron feature etching. Three issues are critical for ECR reactor design: plasma uniformity, ion energy control, and wafer temperature control. Plasma uniformity is important for minimizing over etch times and reducing the probability of producing charging damage. Ion energy control is needed to optimize etching rate, anisotropy, and selectivity without compromising device yield. Wafer temperature control is important because large ion currents at low pressure can result in wafer heating and thereby alter the rates of surface chemical processes. An ECR plasma reactor is described that is designed to etch compound semiconductors and Si at low temperatures (−170 to 20 °C), where superior selectivity and linewidth control are achievable. By measuring dc bias, floating potential, and ion saturation current densities it is shown that ion energies in this system can be controlled by applying an rf bias to the sample. To characterize plasma uniformity, the radial ion density profile is measured using a fast injection Langmuir probe. Hollow, peaked, or uniform radial plasma density profiles can be obtained depending on microwave power, pressure, and magnetic field. Plasma density profiles are influenced by microwave absorption and refraction which, in turn, are influenced by both the magnetic field and plasma density profiles. The net result is a strong coupling between wave propagation and charge particle transport. To control wafer temperature a cryogenic electrode capable of maintaining a wafer temperature to ±2.5 °C at −170 °C is used while the wafer is exposed to an electron cyclotron resonance plasma. The sample temperature is monitored using infrared laser interferometric thermometry and the heat flux to the wafer surface in an Ar plasma is measured as a function of operating parameters by monitoring temperature transients as the discharge is gated on and off.

Negative ions formed at grid surfaces in a flux of excited Cs atoms and ions

In a thermionic energy converter with a carbon layer on the colder electrode, the so-called collector, a thermal plasma with special properties is formed. The flux of Cs from such a converter contains highly excited Cs atoms, even doubly excited states Cs 2★ and negative excited ions (Cs ★) − [J. Lundin and L. Holmlid, Adv. Mass Spectrom., 11A (1989) 204; J. Phys. Chem., 95 (1991) 1029]. In the present experiments, a shielded grid is mounted in this flux and held at a negative potential. Negative ions are formed at the grid and observed by quadrupole mass spectrometry. Ions H − 2, CO − 2, O −, OH −, C − 2 and Mo − are observed as negative peaks in the mass spectra, at current levels up to 0.1 nA. Ion energy analysis is formed by varying the voltage of the collector at the exit of the quadrupole mass spectrometer. This analysis shows that the ions are formed at the negative grid, and experiments with a lens instead of the grid give no negative ions. The grid must have a very low work function to be able to give such large ion currents. It is suggested that the grid surface is covered by a layer of excited states.

Low-energy etching of GaAs using a single-grid ion beam

When microfabricating semiconductor materials, it is necessary to reduce etching damage and to apply a high‐selectivity mask. A single‐grid ion beam etching system can generate much larger ion currents at low accelerator voltages than a dual grid system. In this experiment, a tenfold ion current as that from a dual grid system is extracted at accelerator voltages below 1000 eV. Here, GaAs samples are etched with an Ar ion beam in order to evaluate the physical etching damage, as measured from the I–V curves of a Schottky diode. The normalized n factor, which indicates the amount of etching damage, is less than 1.2 for an accelerator voltage less than 100 eV. The ion current density can be increased without increasing the etching damage. Additionally, several mask materials for GaAs etching are examined. Ion beam sputtered carbon has the highest selectivity, about 15–20 at Va below 150 eV, among the materials examined. The selectivities of Ti for a metal mask and Al2O3 for a transparent mask are more than 10 at Va below 100 eV, whereas the selectivity of SiO2 is 2.6.

Precise age determinations and petrogenetic studies using the KCa method

High precision mass spectrometric determination of calcium isotope ratios allows the 40K → 40Ca radioactive decay to be used for dating a much broader range of geologic materials than is suggested by previous work. 40 Ca 42 Ca is used to monitor enrichments in 40Ca and can be measured to ±0.01% (2σ) using an exponential mass discrimination correction (Russell et al., 1978) and large ion currents. The earth's mantle has such a low K Ca (∼0.01) that it has retained “primordial” 40 Ca 42 Ca = 151.016 ± 0.011 (normalized to 42 Ca 44 Ca = 0.31221 ), as determined by measurements on two meteorites, pyroxene from an ultramafic nodule, metabasalt, and carbonatite. 40 Ca 42 Ca ratios can be conveniently expressed relative to this value as ϵ Ca in units of 10 −4. To test the method for age dating, a mineral isochron has been obtained on a sample of Pikes Peak granite, which has been shown to have concordant KAr, RbSr, and UPb ages. Plagioclase, K-feldspar, biotite, and whole rock yield an age of 1041 ± 32 m.y. (2σ) in agreement with previous age determinations ( λ K = 0.5543 b.y. −1, λ β− λ K = 0.8952 , 40K = 0.01167%). The initial 40 Ca 42 Ca of 151.024 ± 0.016 ( ϵ Ca = +0.5 ± 1.0), indicates that assimilation of high K/Ca crust was insufficient to affect calcium isotopes. Measurements on two-mica granite from eastern Nevada indicate that the magma sources had K/Ca ≈ 1, similar to intermediate-composition crustal rocks. These results show that the KCa system can be used as a precise geochromometer for common felsic igneous and metamorphic rocks, and may prove applicable to sedimentary rocks containing authigenic K minerals. The relatively short half-life of 40K, the non-volatile daughter, and the fact that potassium and calcium are stoichiometric constituents of many minerals, make the KCa system complementary to other dating methods, and potentially applicable to a variety of geologic problems.

Photodissociation of ions within the source of a double-focusing mass spectrometer

Ions generated by electron ionization are photodissociated by shining laser light into the ionization chamber of the source of a conventional geometry, double-focusing mass spectrometer. Photofragment ions formed within the chamber fall through the full potential difference (3 kV) applied between the chamber and the earthed exit aperture. They are detected and recorded using the same procedure as that used to obtain a mass spectrum; the magnetic field is scanned with the electric sector voltage held constant at the value required for transmission of all 3 keV ions. Photofragment ions are distinguished from other fragment ions by using the technique of phase-sensitive detection. The laser beam is mechanically chopped and a lock-in amplifier connected to the detector records only those currents which are in phase with the chopping frequency. Fragment ion spectra are obtained with the full resolution of the double-focusing mass spectrometer and large photofragment ion currents are obtained. The dependences of relative photoabsorption cross-sections and competing dissociation reactions on photoexcitation energy have been studied. The influence of source temperature on competing dissociation reactions of photoexcited ions has been studied by working at one laser light wavelength and several source temperatures. Also, photoionization of Ar + ions using 2.41 eV photons has been demonstrated confirming that these singly charged ions can exist in highly excited, long-lived states.

The Radio-Frequency Quadrupole - A New Linear Accelerator

In many Laboratories, great emphasis now is placed on the development of linear accelerators with very large ion currents. To achieve this goal, a primary concern must be the low-velocity part of the accelerator, where the current limit is determined and where most of the emittance growth occurs. The use of magnetic focusing, the conflicting requirements in the choice of linac frequency, and the limitations of high-voltage dc injectors, have tended to produce lowvelocity designs that limit overall performance. The radio-frequency quadrupole (RFQ) linear accelerator, invented in the Soviet Union and developed at Los Alamos, offers an attractive solution to many of these low-velocity problems. In the RFQ, the use of RF electric fields for radial focusing, combined with special programming of the bunching, allows high-current dc beams to be captured and accelerated with only small beam loss and low radial emittance growth. Advantages of the RFQ linac include a low injection energy (20-50 keV for protons) and a final energy high enough so the beam can be further accelerated with high efficiency in a Wideroe or Alvarez linac. These properties have been confirmed at Los Alamos in a highly successful experimental test performed during the past year. The success of this test and the advances in RFQ design procedures have led to the adoption of this linac for a wide range of applications. The beam-dynamics parameters of three RFQ systems are described.

Measurements of large ion currents in a pinched relativistic electron beam diode

We have observed ion currents of 10–20% of the total diode current in a diode exhibiting pinched electron flow. Measurements have been made utilizing two independent techniques—multiple Faraday cups, and neutron generation and counting. The results agree well with each other and are compared with both numerical simulations and an analytic model of ion flow. Agreement is found to within the accuracy of the experimental measurement, lending further credence to the theoretical models.

Ion-Induced Pinch and the Enhancement of Ion Current by Pinched Electron Flow in Relativistic Diodes

A new model for time-dependent and steady-state ion and electron flow in large-aspect-ratio diodes is constructed. The electron trajectories are computed with use of the self-consistent fields calculated during the initial ion motion. The dynamic formation of a tightly pinched electron flow is qualitatively explained. Very large ion currents, nearly equal to the electron current, are predicted for flat solid cathodes, when steady-state flow is achieved. (AIP)

Sputter cleaning and etching of crystal surfaces (Ti, W, Si) monitored by Auger spectroscopy, ellipsometry and work function change

Surfaces of Ti, W and Si have been cleaned in an ultra high vacuum system by ion bombardment with Cs + and Ar +. Also a thin film (220 Å) of Si 3N 4 on Si has been etched off by sputter-etching with Cs + at 500V. The surface composition was monitored with Auger spectroscopy, film thickness with ellipsometry and work function changes with surface potential difference measurements. Surface contamination on Si, Ti and W samples is mostly C, probably from hydrocarbons used for cleaning or as adsorbed CO. The C and O contamination (∼ 12 Å) on Si 3N 4/Si can be removed with about 6.2 μA-min/cm 2 of bombardment with Cs + at 500 V. At 800 °C no Cs is left on the sample but if the sample is near room temperature, Cs is left embedded in the sample. C and O contanination can be removed from the surface of W and Ti by ion bombardment with Cs + or Ar +. However, at low temperatures these ions are left embedded in the Ti substrate. The embedded Cs or Ar change the work function and ellipsometer values and therefore must be removed by annealing at high temperature. Ti presents the special problem that during the anneal Cl and S segregate and cover the surface even though the bulk concentration is ∼0.1 ppm. A minimum of contamination (<0.1 monolayer) by Ar, Cl and S is obtained by bombarding with Ar + at about 525 °C. Analysis of the composition of thin films can be made by monitoring the surface composition by Auger spectroscopy as the film is sputter-etched away. The sputter-etching efficiency for 500V Cs + to remove Si 3N 4 from Si 3N 4/Si is between 0.55 and 0.73 Å(μA-min/cm 2) or about 4 atoms/ion. Since the ion current ∼ 1 μA/cm 2, it takes about 15 min to remove 10 Å. The conductivity of the SiO 2 is much larger than for the Si 3N 4 and much larger ion currents and sputter-etching rates could be obtained.

A source for the production of large dc ion currents

A source for the production of large ion currents has been developed based on the “duo-plasmatron” principle suggested by M. von Ardenne). Constant hydrogen ion currents in excess of 300 mA have been obtained with accelerating voltages up to 80 kV. The mass ratios of hydrogen ions have been analyzed magnetically with currents up to 100 mA. In this range, up to 70 per cent of the total current could be made H 2 + . The operating characteristics of the source were determined as the various parameters were independently varied.